1. Field of the Invention
The present invention relates to a polishing apparatus for polishing a substrate such as a semiconductor wafer, and more particularly to a polishing apparatus capable of continuously detecting, on a real-time basis, the thickness of an insulating film (layer) or a metallic film (layer) on a surface, being polished, of the substrate in such a state that the substrate is mounted on a substrate holder such as a top ring.
2. Description of the Related Art
In recent years, a higher integration of a semiconductor device requires the narrower wiring and the multilayer wiring, and hence it is necessary to make a surface of a semiconductor substrate highly planarized. This is because the narrower wiring has led to the use of light with shorter wavelengths in photolithography and a tolerable difference of elevation at the focal point on the substrate becomes smaller in the light with shorter wavelengths. Therefore, smaller difference of elevation at the focal point, i.e., higher flatness of the surface of the substrate is necessary.
One customary way of planarizing the surface of the semiconductor substrate is to remove irregularities (concaves and convexes) on the surface of the semiconductor substrate by a chemical mechanical polishing (CMP) process. In this case, after the semiconductor substrate is polished for a certain period of time, the polishing operation is required to be terminated at a desired position or timing. For example, in some cases, an insulating film (layer) of SiO2 or the like is to be left on a metallic wiring of copper, aluminum or the like. Since a metallic layer or other layer is further deposited on the insulating layer in the subsequent process, this insulating layer is called an “interlayer.” In this case, if the semiconductor substrate is polished excessively, the metallic underlayer is exposed on the surface, and hence the polishing is required to be terminated in such a state that a predetermined thickness of the interlayer remains unpolished.
Further, in some cases, interconnection grooves for a predetermined wiring pattern are formed in a semiconductor substrate, conductive materials such as copper (Cu) or copper alloy are filled in such grooves of the semiconductor substrate, and then unnecessary portions of the conductive materials on the surface of the semiconductor substrate are removed by a chemical mechanical polishing (CMP).
When the copper layer is polished by the CMP process, it is necessary that the copper layer on the semiconductor substrate be selectively removed therefrom, while leaving only the copper layer in the grooves for a wiring circuit, i.e. the interconnection grooves. More specifically, the copper layer on those surface areas of the semiconductor substrate other than the interconnection grooves needs to be removed until an oxide film of SiO2 or the like is exposed. If the copper layer in the interconnection grooves is excessively polished away together with the oxide film such as SiO2, then the resistance of the circuits on the semiconductor substrate would be so increased that the semiconductor substrate might possibly need to be discarded, resulting in a large loss. Conversely, if the semiconductor substrate is insufficiently polished to leave the copper layer on the oxide film, then the circuits on the semiconductor substrate would not be separated from each other, but short-circuited. As a consequence, the semiconductor substrate would be required to be polished again, and hence its manufacturing cost would be increased. This holds true for semiconductor substrates which have an electrically conductive layer of aluminum or the like that needs to be selectively be polished away by the CMP process.
Therefore, it has been proposed to detect an end point of the CMP process using an optical sensor. In such end point detecting process in the CMP process, an optical sensor comprising a light-emitting element and a light-detecting element is provided adjacent to the turntable. A top ring for holding a semiconductor substrate is moved laterally to protrude the semiconductor substrate from the outer circumferential edge of the turntable, thereby exposing the surface, being polished, of the semiconductor substrate. In this state, the light-emitting element applies light to the surface, being polished, of the semiconductor substrate, and the light-detecting element detects reflected light from the surface of the semiconductor substrate to thus measure the thickness of the insulating layer or the metallic layer on the surface of the semiconductor substrate and detect the end point of the CMP process.
However, this method is problematic in that during polishing of the semiconductor substrate, the thickness of the insulating layer or the metallic layer on the surface, being polished, of the semiconductor substrate cannot be measured at all times.
Further, in the case where the thickness of the layer is measured over a position ranging from the outermost periphery to the center of the semiconductor substrate according to the above detecting process, the protrusion of not less than 50% of the surface of the semiconductor substrate from the turntable is necessary. In this case, since the top ring has a universal joint such as a gimbal mechanism so as to follow the inclination of the polishing surface on the turntable, the top ring is inclined and the semiconductor substrate is hit against the outer peripheral edge of the turntable to cause breaking or damaging of the semiconductor substrate.